Patent GB-887660-A (GB)
Title: Fire detectors
IPC: G08B17/06
Filed: 1959-06-19
Abstract: 887,660. Electric fire alarm systems; temperature alarms. LEMAIRE, F. C. June 19, 1959 [June 21, 1958; Dec. 30, 1958], No. 21133/59. Class 118. [Also in Groups XL (b) and XL (c)] A circuit for detecting rate of change of temperature, comprises a temperature sensitive resistance in series with a fixed resistance connected across a source of direct voltage, a capacitor-resistor differentiating circuit with the capacitor connected to the junction of the first two resistances and the resistor connected to a point of constant potential, and a diode connected from the junction of the capacitor and resistor of the differentiating circuit to a second point of constant potential, the arrangement being such that a rate of rise of temperature exceeding a predetermined value biases the diode to cut it off. A device as described is used to initiate a fire alarm signal and may be connected, together with a number of other such devices to a central point where an indication is also provided of the particular device initiating the alarm. Distinct indications are also provided of faults in the system. Sensing element (Figs. 1, 6).-The sensing element comprises a semi-conductor element, e.g. a thermistor Th, mounted in a copper or aluminium heat collector C. The thermistor is connected in series with the resistor r across a source of potential, the junction of R and Th being connected through a capacitor C to a second resistor r and a diode D. The diode is biased to be normally conductive so that the potential at point x is substantially Eo. For slow rates of change of potential at M the current produced through Cr is insufficient to cut off D and the potential at x is substantially constant, but for high rates of change, D cuts off and the potential at x can change to operate an alarm. A rectifier may be connected across the capacitor such that for large variations in potential at point M the capacitor is shunted and the alarm is operated by the change in voltage instead of the rate of change. Fire alarm (Fig. 14).-The output from the point x is fed to a two-stage transistor amplifier Ta, Tb to operate a polarized relay P over winding I. Operation of this relay causes relay RAS to operate and lock in to give a fire alarm warning over AS. Fault alarm.-If the sensing element becomes either open circuit or short circuit, the sudden change in potential at point M (either up or down) will operate either of transistors T1 or T2 in addition to transistors Ta, Tb and feed a signal via bridge B to winding II on relay P. The combined ampere-turns of the two windings are arranged to be such that the relay is magnetized in the opposite sense and operates a fault on alarm relay RAT. In an alternative embodiment (Fig. 13, not shown), the bridge B and winding II are not provided, each transistor T1, T2 having instead a relay in its collector circuit which operates to give an open circuit or short circuit alarm. Multiple alarm system (Fig. 16).-A number of sensing elements are connected in parallel and are fed into the central unit via or-gates comprising diodes Sik, Gk, &c. A fire alarm initiated by element Thk causes relay P to operate as described before, this operates relays relays P1 and C0 to connect the sensing element to the transistor T1k and operate relay RASk which indicates the element that has initiated the alarm, and also operates alarm relay RAS referred to previously. Operation of RASk insolates the element initiating the alarm from the relay CO so that when relay P, P1 C0 restore, the element is isolated from the alarm amplifier Ta, Tb. A fault alarm magnetizes relay P in the opposite sense as described above and hence relays P2, RATk, and RAT operate instead of relays P1, RASk, RAS to give a fault alarm, and to isolate the faulty element from the system in a manner similar to that described above.